The Navy, Air Force and Coast Guard have used crossbar-type indicators to present doppler radar-derived ground speed cues to helicopter pilots. That is an inexpensive type indicator available in large quantities in the government supply system.
That indicator format provides the pilot with omnidirectional information. The intersection of the two bars tells the pilot which direction the helicopter is moving. A line between the intersection of the two bars and the zero speed point portrays the direction of travel of the craft. The bars of that type of indicator provide very strong visual cues which can provide useful piloting information even in a peripheral scan.
There are two approaches that are used when connecting speed sensors to bars of the indicators. In one approach, when forward speed is generated, the horizontal bar moves downscale, and when right sideward speed is generated the vertical bar moves to the left. That is the display technique that doppler installations have used for over 15 years.
There is a second approach which may be more desirable for Marine Corps and Army applications. In this alternative, the horizontal bar moves upscale to indicate forward speed, and the vertical bar moves to the right to indicate sideward flight toward the right.
Helicopter speed displays are found in U.S. Patent and Trademark Office in Class 73, Measuring and Testing, Sub-Class 178 H and in Class 116 Indicators, Sub-Class 129 and Digest 43. Examples of prior art are U.S. Pat. Nos. 2,567,212; 2,845,623; 3,048,836; 3,395,576; and 3,355,733.